Keep Only One-Off Values in a List

Medium

You get a singly linked list starting at head; its values come in no particular order. Find every value that occurs more than once in the list and delete **all** nodes holding such values. The remaining nodes keep their order. Return the head of the resulting list (an empty list if no nodes are left).

Example 1

head=list_node([4, 7, 4, 9, 1]) → [7, 9, 1]

The value 4 occurs twice, so both nodes holding 4 are removed. What remains is 7 → 9 → 1.

Example 2

head=list_node([5, 5, 3, 3, 3]) → None

Every value repeats, so the list becomes empty.

Example 3

head=list_node([8, 2, 6]) → [8, 2, 6]

Constraints

1 ≤ number of nodes ≤ 10⁵
1 ≤ Node.val ≤ 10⁵

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Topics

Hash Table Linked List

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Input

head=list_node([4, 7, 4, 9, 1])

Expected

[7, 9, 1]

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